As an example of a wireless communication system to which the present invention is applicable, a 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) communication system will be briefly described.
FIG. 1 is a view showing a network structure of an evolved universal terrestrial radio access network (E-UTRAN) which is an example of a mobile communication system. The E-UTRAN system is a system evolved from the existing UTRAN system and a basic standardization thereof is currently being performed in the 3GPP. The E-UTRAN system may be called a long term evolution (LTE) system.
The E-UTRAN includes eNBs (e-NodeBs or base stations) and eNBs are connected via an X2 interface. An eNB is connected to a user equipment (UE) via an air interface and is connected to an evolved packet core (EPC) via an S1 interface.
The EPC includes a mobility management entity (MME), a serving-gateway (S-GW) and a packet data network-gateway (PDN-GW). The MME has access information of a UE and information about capabilities of the UE. Such information is mainly used for mobility management of the UE. The S-GW is a gateway having an E-UTRAN as an end point and the PDN-GW is a gateway having a PDN as an end point.
Layers of a radio interface protocol between a UE and a network may be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based on the three lower layers of an open system interconnection (OSI) standard model which is well-known in the art of communication systems. A physical layer, which is the first layer, provides an information transfer service by using a physical channel. A radio resource control (RRC) layer located at the third layer serves to control radio resources between a UE and a network. For control of radio resources, the RRC layer exchanges an RRC message between the UE and the network.
Although radio access technology has been developed to LTE based on WCDMA, the demands and expectations of users and providers have been lastingly increased. In addition, since other radio access technologies have been developed, new technological evolution is required for securing high competitiveness in the future. Decrease in cost per bit, increase in service availability, flexible use of a frequency band, simple structure, open interface, suitable UE power consumption and the like are required.